The use of an intermediate soft sealing glass to join preformed glass parts, which sealing glass has a softening point sufficiently below that of the glass being sealed to permit the formation of a seal without any distorting flow of the preformed parts, has been recognized in the glass art for more than 40 years. U.S. Pat. No. 2,642,633 (Dalton) is illustrative of that practice. Hence, one method described therein comprised powdering (fritting) a glass, forming a slurry of that powder in a vehicle, and then flowing, brushing, or spraying the slurry onto the surfaces to be joined together. Thereafter, the parts were brought together and heated sufficiently to cause the layer of frit to flow, but not high enough to soften the glass parts themselves. The frits consisted essentially of PbO, Al.sub.2 O.sub.3, B.sub.2 O.sub.3, and up to 10% SiO.sub.2. Dalton described the use of those sealing glasses to bond the faceplate to the funnel of a cathode ray tube.
Soft glasses of the type disclosed by Dalton frequently experienced problems when used in sealing cathode ray and other electronic tube parts. Thus, it is customary to bake out the tube, that is, to evacuate the tube while subjecting it to an elevated temperature viz., temperatures up to as high as about 500.degree. C., to remove absorbed gases. Whether the bakeout operation is conducted concurrently with the sealing operation or is a separate subsequent step, the soft glasses were prone to become soft and flow at the temperatures involved, and, under the influence of a pressure difference, tended to slip between the sealed components. The soft glasses were also inclined to bubble or foam under vacuum and were frequently quite sensitive to heat shock.
Those problems were corrected through the development of thermally devitrifiable sealing glasses, the genesis of which is disclosed in U.S. Pat. No. 2,889,952 (Claypoole). As explained therein, the sealing glass is fritted, mixed with a conventional organic binder and vehicle to form a slurry, suspension, or paste, which is then applied to the surfaces of the parts to be joined together. The coated parts are assembled, the assembly heated to a predetermined temperature above the softening point of the glass (with either an adequate break in the heating cycle or at a sufficiently slow heating rate to assure burnout of the organic materials) to fuse the glass powders to wet the sealing surfaces and flow into the desired seal configuration, and the fused sealing glass thereafter heated at its devitrification temperature for a sufficient length of time to cause devitrification of the glass. The glass will customarily be devitrified at a temperature at least as high as the temperature at which the fused glass seal is formed.
The patent describes glass compositions within the base PbO--ZnO--B.sub.2 O.sub.3 system as being especially suitable for use as devitrifible sealing glasses in sealing the panel to the funnel of a cathode ray tube, and the use of a combination of nitrocellulose and amyl acetate as the vehicle and binder, respectively. British Patent No. 863,500 disclosed specific composition intervals of devitrifiable sealing glasses within the base PbO--ZnO--B.sub.2 O.sub.3 --SiO.sub.2 system, which intervals are recited below in weight percent on the oxide basis:
______________________________________ PbO 75-82 B.sub.2 O.sub.3 6.5-12 ZnO 7-14 SiO.sub.2 1.5-3. ______________________________________
Up to 3% Al.sub.2 O.sub.3 may advantageously be present. Other optional components totalling no more than 5% include 0-5% CdO and/or Fe.sub.2 O.sub.3, 0-4% BaO, 0-1% Li.sub.2 O and/or Na.sub.2 O, 0-1% As.sub.2 O.sub.3, 0-1% Sb.sub.2 O.sub.3, and 0-1% colorants.
U.S. Pat. No. 5,470,804, filed by Robert M. Morena Aug. 3, 1994 under the title MILL ADDITIONS FOR SEALING GLASSES and assigned to the same assignee as the present application, discloses sealing glass materials comprising thermally crystallizable, PbO--Zno--B.sub.2 O.sub.3 --SiO.sub.2 glass frit and a mill addition selected from the group consisting of Al.sub.2 O.sub.3, zircon, and MnO.sub.2, the mill addition being present in an amount sufficient to increase the mechanical strength in a fusion seal to at least 55.2 MPa (8000 psi), but the amount being not over about 5% by weight of the sealing material.
Currently two PbO--ZnO--B.sub.2 O.sub.3 --SiO.sub.2 base glass frits are commercially available from Corning Incorporated, Corning, N.Y., under Code Numbers 7580 and 7590. Corning Code 7580 is a vitreous sealing glass frit having the following approximate composition expressed in terms of parts by weight on the oxide basis, of
______________________________________ PbO 74.4 B.sub.2 O.sub.3 8.4 BaO 1.9 ZnO 12.9 SiO.sub.2 2.2 Al.sub.2 O.sub.3 0.03 ______________________________________
Corning Code 7590 comprises the base glass of Code 7580 frit with about 1% by weight zircon as a mill addition to the frit. The zircon mill addition acts as a crystallization catalyst in the Code 7580 frit. It serves to initiate crystallization late in a sealing process.
In recent years pressure has been continuous to remove lead from glass compositions. That pressure has led to laboratory investigations to develop lead-free frit compositions suitable for use as sealing glasses. One composition area which has provided lead-free devitrifiable frits suitable for joining component parts in electrical and electronic devices and for sealing the panel to the funnel of a cathode ray tube comprises tin phosphate-based glasses. The following are illustrative of such glasses.
U.S. Pat. No. 5,246,890 (Aitken et al.) discloses SnO--Zno--P.sub.2 O.sub.5 glasses consisting essentially, in mole percent on the oxide basis, of 25-50% P.sub.2 O.sub.5 plus SnO and ZnO in amounts such that the mole ratio of SnO:ZnO is 1:1 to 5:5. Optionally, up to 20% total of modifying oxides such as up to 5 mole % SiO.sub.2, up to 20 mole % B.sub.2 O.sub.3, and up to 5 mole % Al.sub.2 O.sub.3, as well as one or more crystallization promoters selected from the group of 1-5 mole % zircon and/or ZrO.sub.2 and 1-15 mole % alkali metal oxides, may be included. The glasses are stated to be especially suitable for joining component parts in articles such as cathode ray tubes.
U.S. Pat. No. 5,281,560 (Francis et al.) reports tin phosphate-based glasses consisting essentially, in mole percent on the oxide basis, of 25-50% P.sub.2 O.sub.5, 30-70% SnO, 0-15% ZnO, the mole ratio SnO:ZnO being greater than 5:1, and at least one oxide being present in an amount effective to stabilize the glass against devitrification and/or exudation during sealing up to 25% total in the indicated proportion selected from the group of 0-25% R.sub.2 O, wherein R.sub.2 O consists of 0-25% Li.sub.2 O, 0-25% Na.sub.2 O and 0-25% K.sub.2 O, 0-20% B.sub.2 O.sub.3, 0-5% Al.sub.2 O.sub.3, 0-5% SO.sub.2, and 0-5% WO.sub.3. These glasses are stated to be softer, that is, capable of forming seals at lower temperatures, than the Aitken et al. compositions, supra.
U.S. application Ser. No. 08/221,400, filed Mar. 31, 1994 by Robert M. Morena under the title FUSION SEAL AND SEALING MIXTURES discloses a sealing material comprising 60-90% by weight of a SnO--ZnO--P.sub.2 O.sub.5 glass frit and 10-40% by weight of a mill addition including 10-30% Al.sub.2 O.sub.3, 0-30% zircon, and 0-15% of a further additive which reduces the effective linear coefficient of thermal expansion of the seal. The glass frit consists essentially, in mole percent on the oxide basis, of 25-50% P.sub.2 O.sub.5 plus SnO and ZnO in a molar ratio of 5:1 to 2:1. The glass frit may optionally contain up to 20 mole % total of at least one modifying oxide selected from the group of 0-5 mole % SiO.sub.2, 0-20 mole % B.sub.2 O.sub.3, 0-5 mole % Al.sub.2 O.sub.3, and 0-5 mole % WO.sub.3.
Lead-containing sealing glass frits are known in the prior art as being prepared with binders and vehicles (the combination of those two components being commonly referred to as lacquer) with the addition of a compatible surfactant to provide an acceptable paste. For example, lead-containing sealing frit pastes are frequently formulated utilizing a lacquer comprising a nitrocellulose/amyl acetate mixture which, through the incorporation of a compatible surfactant, yields acceptable pastes. Those pastes, however, settle and demonstrate inconsistent viscosities (resistance to flow as a function of deformation rate) and yield values (resistance to flow at zero deformation rate) with time. Thus, the lead-containing frit paste is remixed before application and the inconsistent paste viscosities are adjusted by varying the frit/vehicle ratio and/or the amount of surfactant employed. These production and application difficulties are exacerbated in the above newly developed lead-free frits primarily because of differences in the compositions and densities of the lead-containing and lead-free frits. The ratio of densities of the lead-free to the lead-containing frits is about 0.6.
Therefore, the principal objective of the present invention was to develop sealing paste formulations which are substantially non-settling and exhibit consistent viscosities and yield values, thereby improving product performance and reproducibility.
A specific objective was to develop sealing paste formulations that are useful with both lead-containing and lead-free frits.